Typically, radiotelephones include a main body having a receiver or speaker housed in an upper portion thereof and oriented to direct sound directly through an outer housing into the user's ear. The overall quality of speech and sound emitted by the receiver of a radiotelephone is sometimes referred to as speech intelligibility. It is an objective in devices such as radiotelephones to minimize size and weight without compromising speech intelligibility.
Speech intelligibility in radiotelephones is generally affected by a number of design factors including the receiver design, how and where the receiver is housed, and how the emitted sound energy is channeled to the ear. Ideally, a radiotelephone should produce a fairly level frequency response for a frequency range of approximately 300 Hz to 3000 Hz.
Factors such as interference from electrical components in radiotelephones presents restrictions on the design of receivers. U.S. Pat. No. 5,963,640, the disclosure of which is incorporated by reference, discloses the use of an acoustical waveguide coupled to a speaker to permit mounting of the speaker at a point remote from sound ports that are intended to be placed proximate a user's ear and thus increase design flexibility. Nonetheless, receivers are designed in view of the impedance of free air, certain design features available in larger receiver systems such as loudspeakers and conventional telephones are not adapted to be used in receivers for radiotelephones. The impedance of free air necessitates that large volumes of air be moved to generate sufficiently intelligible sound. Accordingly, relatively large drivers are necessary to match the impedance of free air. This, in turn, presents limits in terms of the size, weight, and type of equipment that can be used, and how and where it can be located in a radiotelephone.
For example, although impedance matched horns matched to the impedance of the driver and free air are quite effectively used in large systems, the impedance of air requires such horns to be of sizes too great for use in smaller devices such as radiotelephone receivers. While various prior art radiotelephone devices, such as those disclosed in U.S. Pat. No. 5,915,015, U.S. Pat. No. 5,832,079, and WO98/51122, the disclosures of which are incorporated herein by reference, disclose the use of horns in connection with microphones, providing impedance matched horns in these devices must necessarily involve matching impendence with the impedance of free air as the speaker's mouth is typically held at a distance from the mouthpiece apertures. Accordingly, providing a horn arrangement in such devices presents a restriction on the size of the radiotelephone. U.S. Pat. No. 5,832,079 discloses that acoustic reciprocity dictates that the disclosed combination of an impedance matched horn and microphone is equally applicable to routing of output sound from interiorly mounted electromagnetic transducers to an output acoustic horn. However, such a horn and transducer would be of substantial size, just like the disclosed horn and microphone.
The present invention solves problems associated with prior art systems through the inventor's recognition that impedance of free air is not necessarily an appropriate design factor for consideration in radiotelephones. More particularly, the present invention relates to the inventor's discovery that, in receivers, it may be appropriate to match impedance of a horn to impedance of the driver and impedance of a user's ear instead of matching impedance to impedance of free air. This discovery results in permitting substantially smaller impedance matched horns and drivers than were previously known. Because the size of receivers including drivers and impedance matched horns can be substantially reduced according to the present invention, radiotelephone design flexibility is substantially improved, as the receivers are more easily isolated from potentially interference-using components, and the weight and size of radiotelephones can be reduced. Further, speech intelligibility need not be compromised and may be enhanced through the use of a receiver including a horn having impedance matched with the impedance of an ear.
In accordance with one aspect of the present invention, an acoustic horn has an acoustic impedance matched with impedances of an ear and a driver.
In accordance with another aspect of the present invention, an electroacoustic transducer includes a driver and an acoustic horn having an acoustical impedance matched with impedances of an ear and the driver.
In accordance with yet another aspect of the present invention, a portable device includes a body, a driver mounted inside the body, and an acoustic horn having an acoustical impedance matched with impedances of an ear and the driver, a large end of the horn extending to a position proximate an exterior surface of the body.
Beyond the patent literature, the published technical literature also provides useful description regarding the desirable attributes of exponential horns for sound propagation. In particular, a 1924 A.I.E.E. article authored by C. R. Hanna et al. is of interest for its teaching of the theoretical considerations pertaining to the design of acoustic horns for best sound propagation.